Machines such as, for example, excavators, loaders, dozers, motor graders, and other types of heavy equipment use multiple actuators supplied with hydraulic fluid from an engine-driven pump to accomplish a variety of tasks. These actuators are typically pilot controlled such that, as an operator moves an input device, for example a joystick, an amount of pilot fluid is directed to a control valve to move the control valve. As the control valve is moved, a proportional amount of fluid is directed from the pump to the actuators. Various hydraulic control strategies have been implemented to control the amount of fluid flow between the pump and the actuators, including a load sensing control strategy. Load sensing control strategies measure a pressure differential between a maximum load pressure of a plurality of actuators and a pump delivery pressure. A controller typically receives the pressure differential data and controls a displacement of the pump to deliver the maximum load demand. More specifically, load sensing control systems attempt to control pump displacement to maintain a desired buffer pressure between pump delivery pressure and the maximum load pressure. Since variable displacement pumps are known to react slowly to load pressure changes, the pump is typically controlled to deliver fluid at an excessive pressure to ensure the maximum load pressure is available to the actuators. Hence, the pump is often required to deliver more pressure than necessary to overcome its own slow response to load demands.
One example of such a load sensing control system has been described in U.S. Pat. No. 5,129,230 (the '230 patent) to Izumi et al. Specifically, the '230 patent discloses a hydraulic control system implementing a variable displacement pump, two cylinders, two control valves, and an unloading valve. Additionally, the '230 patent discloses a load pressure sensor for sensing the maximum load from the two cylinders, and a pump swash-plate position detector. Based on the sensed values from the load pressure sensor and the swash-plate position detector, a pressure difference between the pump and the maximum load is determined and transmitted to a controller. The controller instructs the variable displacement pump to deliver an excessive amount of pressure to ensure that the pump delivery pressure is greater than the maximum load pressure. An unloading valve is positioned between the pump and the control valves for holding the differential pressure less than a setting value. As a result, the '230 patent is able to control a delivery rate of the pump when there are small or large pressure differences between the pump and the maximum loads.
Although load sensing pump control may, by itself, be adequate for some situations, at time it may be limited and inefficient. That is, pump control may be slow to respond to changes in required load pressure. And, pump control systems must maintain a relatively high amount of pressure differential to ensure that pump pressure is sufficient to meet the needs of the maximum load. These high pressures may place an unnecessary strain on the machine, whereby causing the pump to be overworked and the power source to inefficiently use fuel.
The disclosed actuator control system is directed to overcoming one or more of the problems set forth above.